UHF/GMRS Question

[ButlerAlerts]

Ok, so I have been doing some VHF Stuff every now and then. But I got into UHF/GMRS a few years back, for just receiving, not transmitting. So here is my question to the more knowledgeable guys on this forum.

I know UHF is considered Line Of Site, but that dont mean you have to see the other antenna, just that it is in a straight sight since UHF transmits straight lines, and that the wavelength is shorter than VHF, with a 1.5 Foot Clearance. Would I be able to hit a GMRS Repeater about 11-12 Miles away on UHF.

These are the factors:
Radio: CDM 1250 40 Watts
Repeater: CDM 1250 40 Watts
My Antenna: A99 (The tip is 38 Feet above ground)
Foliage: Yes
Hills: Yes(Not Severely)

Elevation:
Mine: 900 Feet above Sea Level
Repeater: 1300 Feet above Sea Level (Its on an extremely large hill above a big city)

I know our Fire Department, which has a CDM 1250 can talk to our 911 Center, which is about 14-15 Miles away, with larger hills, and we get a fine signal and audio.

Elevation:
Our FD STation: 900 Feet
911 Center: 1295 Feet
So roughly the same.

I feel I have no problem hitting the repeater, but I do not want to waste over $200 for a radio if I cannot hit it. I know UHF has lots of variables, but please answer these 2 questions.

1. Do you feel I can hit it, Yes or No.
2. Whats was the furtherest UHF repeater you have ever hit, please include the terrain and how much power you used, and how many miles away.

Thank you, and 73s
WQRF973

 

[mmckenna]

A99 antenna? That's a CB/10 meter antenna, and while it may be working, it's not going to be resonate on UHF.

When I used to use GMRS for family communications, I could hit repeaters all over the place, it all depends on location. I'd have no problem hitting repeaters 50 or more miles away if I was in the right location.
Your exact location and the exact location of the repeater is going to determine what you can reach. Without having intimate knowledge of your location, I won't guess.
The fact that the fire station can reach the 911 center doesn't tell us a whole lot. Is that simplex or using a repeater? Where is the repeater located relative to the fire station and the 911 center. Are they on UHF or some other band?

There are propagation mapping programs that could give you a fairly accurate answer, but you'd need to enter fairly accurate latitude/longitude coordinates.

It's really going to depend on your exact situation. 15 miles should be no problem for UHF if there isn't a lot of topographical shielding between you and the repeater. I've done over 15 miles vehicle to vehicle with simple 1/4 wave antennas no problem. Then again, there have been locations where it was impossible to talk a few miles due to topographical shielding.

While UHF is predominately "line of sight", it will bend and reflect.

$200 isn't a bad price for a CDM1250 if it's in good shape. You really are going to need a UHF antenna though, using a CB antenna isn't going to do your radio any favors at all.

 

[ButlerAlerts]

Our 911 Frequencies operate on UHF. The repeater is the 14-15 miles away, located at the 911 center. Our county only uses repeaters, no simplex unless on-scene. There will be foliage and some hills directly to the repeater I am trying to hit. Thanks for the reply though!

 

[Logan005]

I would love to jump in here, but the topography here in Florida is clearly not the same. I have some question though. This repeater you wish to key up. can you hear it on your scanner? when you are close to the repeater, can you key it up with a handheld? is it UHF, or an actual GMRS repeater? I can tell you a good antenna and LMR400 or better cable is critical when transmitting for distance propagation. if a dipole does not work, try a yagi.

 

[mmckenna]

Our 911 Frequencies operate on UHF. The repeater is the 14-15 miles away, located at the 911 center. Our county only uses repeaters, no simplex unless on-scene. There will be foliage and some hills directly to the repeater I am trying to hit. Thanks for the reply though!

Are the hills directly blocking line of site from your antenna to the repeater? Foliage likely won't be an issue.

 

[gewecke]

As said, ditch the cb antenna and get one which is resonant on 462 - 467mhz. you have good equipment listed but it's not going to transmit long with a improper antenna.

73,
n9zas

 

[sandiegosand]

Have you tried to hit it? If the place where the repeater is is a good deal higher than whatever hills are between you, you still should have not much trouble. The repeaters in my area i can hit pretty easily even though there are mountains between, i have hit a repeater 80-90 miles away at just under 7000 ft with only 1.5 watts of power from a handheld. The ones i use the most are about the same distance as the one you described, and there are plenty of hills between. One last thing sometimes you can hear a repeater but cannot hit it, usualy when the repeater output is kickin 100 watts.

 

[SCPD]

I would get a yagi antenna which is a direction antenna and you may find on eBay for a minimal cost or buy from a radio shop,either way tell whomever you purchase from that you need one in the 460-470 MHZ range and buy an adapter with a small length of RG 58 coax to use for test purposes only that connects to your hand held so you can and walk around in your yard and check for best area of reception and employ someone else on the repeater to help you with signal reports.

Find that good location and adjust the antenna for the best transmit/receive signal and nail it down.Also find a friend or amateur radio operator with a antenna analyzer or VSWR meter to assist you in making sure the antenna is tuned and working properly.BTW for the best results as mentioned earlier use Times Microwave LMR 400 coax avoid using a knock off version for there is a difference in which coax you choose.

Be aware you may be able to receive the signal full scale and still not be able to access the repeater its just trial and error,you may go 15 ft high on a mast or 50ft being there is no case where all will work the same for everyone.

 

[rapidcharger]

One thing you might try is zooming into your location on Google Earth (Earth, not Maps) and spin around and see if you can see the hill the repeater is located on. If you've got other hills in the way, I'd be concerned about that. If you've got a mile or less of obstructions such as trees and houses, I would give it a try. At least that's the rather conservative rule of thumb I go by.

There's UHF repeaters that I get into on a regular basis that are 3 times the distance away or more as the one you're trying to hit and I get in on 5 or 25 watts with similar antenna height although I do have a pretty favorable elevation. The elevation is reversed from your situation but with about half the spread.

One of the repeaters I get into with a 4 watt portable with rubber duck antenna from 35 miles away but that's a DMR system so it's a different animal.

 

[KR7CQ]

One thing you might try is zooming into your location on Google Earth (Earth, not Maps) and spin around and see if you can see the hill the repeater is located on. If you've got other hills in the way, I'd be concerned about that.

And I thought that was just my little trick, lol. I have set up a lot of antennas doing that over the years, and Google Earth can be quite telling when you set the horizon as your view and look at the hills in the way.

In the upper part of the 420-470 MHz band, some interesting things can happen with refraction, reflection, etc. I have seem some some signals get through that shouldn't, but by the same token if you have hills in the line of site, you probably have a slim chance of quieting a repeater with an omni-directional antenna (though you may hear the repeater decently), and a better chance of reasonable quieting with a beam antenna. Good luck.

 

[ButlerAlerts]

Ok, I just did that, and there are about 2-3 hills about 100 feet higher then my antenna, but my current antenna is a andtenna with 3 "radiation sticks" the point out on the bottom, with a striaght stick up, with 2 little rods the "cradle" it. And its meant for Hi-VHF and UHF. I think I may be able to hit ti since its 11 miles away via air, and 60 watts of power!lol

 

[KR7CQ]

Ok, I just did that, and there are about 2-3 hills about 100 feet higher then my antenna, but my current antenna is a andtenna with 3 "radiation sticks" the point out on the bottom, with a striaght stick up, with 2 little rods the "cradle" it. And its meant for Hi-VHF and UHF. I think I may be able to hit ti since its 11 miles away via air, and 60 watts of power!lol

Are you by any chance talking about the old Radio Shack "tri-band" ground plane scanner antenna with VHF-Low, VHF-High, and UHF verticals?

If so, I would definitely not recommend that for GMRS TX. For one thing it would have to be close to 20 years old if memory serves correct, it wasn't the greatest antenna to begin with, and it is not tuned for your frequency range. Your radio will either eat itself alive, or if it is more intelligent, it will lower power output to reduce heat in the unit from reflected RF, meaning you won't "get out" very well. I can't imagine you would have a decent SWR with that antenna, but you will only know by testing I suppose. Just be cautious. Yagi directional antennas are really cheap for UHF on ebay. You might want to consider that if you are mainly interested in that one repeater.

If you do need an omni-directional antenna by any chance, look for a collinear ground plane antenna. I had one years ago when I ran a GMRS station with a 50' tower, and I could talk between the home station and my handheld for up to 12-14 miles in a big city with a lot of buildings and obstacles. It was truly a great antenna, wish I could remember the brand.

 

[mmckenna]

You need a proper antenna. End of discussion. What you have described are NOT proper antennas. You are either going to be running a high SWR which will eventually toast your radio, or you are going to radiate so poorly that you just might as well use a FRS radio.
Do not overlook the importance of your antenna system. It doesn't matter how much you spend on the radio, if you don't have a good antenna system, it's not going to work very well.

The only way to really know if you can bring up the repeater in this situation is to try it. If you are investing in the CDM1250, you will need a proper antenna. Cutting corners here isn't going to work. Put up the right antenna with decent feed line and give it a try. If it works but it's scratchy, then try a beam antenna. If it doesn't work at all, then it's possible that no amount of antenna gain is going to overcome the topographical shielding.

Really, you need to understand the antenna system as a whole. Using a scanner or CB antenna is going to lead to disappointment at minimum, damaged equipment at worst.

 

[rapidcharger]

When it comes to UHF, 2-3 hills in the way is going to be a problem for you. A big problem.
I would not expect to get in to the repeater in that case.

 

[ButlerAlerts]

rapidcharger, I couldnt get very far, so im selling it to a local fire department.

 

[WB4CS]

rapidcharger, I couldnt get very far, so im selling it to a local fire department.

I'm curious, did you try it with a correct UHF antenna or did you give up while only still using the A99 HF antenna? You mentioned in another post that the radio is 60 Watts (or 40, you say both ). As mentioned several times, with an antenna that's not resonant your SWR will be very high. Your 40 Watts going to the antenna may only be putting out 5 Watts of actual power, with the other 35 Watts being reflected back into the radio as heat due to high SWR.

I do believe it's possible to accomplish the range you're looking for with a resonant antenna, up high enough, with low loss coax, and running 40 Watts of actual output power. An antenna with 3dB or more of gain should do the trick.

 

[BJ_NORTON]

You might try a program called radio mobile. It plots out predicted radio coverage for a specific transmitter. The desktop version is quite complicated, but the author has made an online version that is quite simple to use. Is meant to be freeware for the ham community so the online version can only plot for frequencies in the amateur bands, but I think the predicted coverage at 450 is going to be close enough for your GMRS setup.

Radio Mobile WEB Site

 

[SCPD]

rapidcharger, I couldnt get very far, so im selling it to a local fire department.

What is a fire dept going to do with a GMRS repeater? They can't legally use it unless all the fire dept personnel are licensed,they can't get one license and use it either...........

 

[gewecke]

What is a fire dept going to do with a GMRS repeater? They can't legally use it unless all the fire dept personnel are licensed,they can't get one license and use it either...........

Well ... IF the op chooses not to use the correct antenna for it, and IF a fire dept was to use UHF for dispatch or some other purpose then they could reprogram that repeater for the licensed frequency THEY use.

73,
n9zas

 

[SteveC0625]

What is a fire dept going to do with a GMRS repeater? They can't legally use it unless all the fire dept personnel are licensed,they can't get one license and use it either...........

Well ... IF the op chooses not to use the correct antenna for it, and IF a fire dept was to use UHF for dispatch or some other purpose then they could reprogram that repeater for the licensed frequency THEY use.

73,
n9zas

Actually, the OP bought a CDM1250 mobile which he wanted to use to hit a repeater some distance away. That repeater coincidentally is also a CDM1250. The OP had other questions in a thread in the Motorola sub-forum which pretty much explained to the rest of us that he doesn't understand UHF (or any) radios at all. He may have figured that out, too, and sold his mobile to the FD. All FD operations in his country are UHF according to the RR database.